In recent years, an increasing number of patients is suffering from respiratory diseases such as asthma, pulmonary emphysema, chronic bronchitis, etc. One of the most effective therapies for these diseases is oxygen inhalation therapy. Such oxygen inhalation therapy makes the patient inhale oxygen gas or oxygen-enriched air. An oxygen concentration device, liquid oxygen, an oxygen gas cylinder, etc. are known as the supply source, among which the oxygen concentration device is mainly used for home oxygen therapy due to its convenience in using and easiness in maintenance.
The oxygen concentration device concentrates oxygen present in the air at about 21% and supplies an oxygen-enriched gas. The oxygen concentration device includes a membrane-type oxygen concentration device utilizing a membrane which selectively permeates oxygen and a pressure-swing adsorption type oxygen concentration device utilizing an adsorbent which is capable of preferentially adsorb nitrogen or oxygen. The latter is mainly used because of the advantage that high-concentration oxygen of 90% or more can be obtained.
The pressure-swing adsorption type oxygen concentration device can continuously generate high-concentration oxygen-enriched gas by alternately repeating a pressurization/adsorption step to obtain an unadsorbed oxygen-enriched gas, wherein nitrogen is adsorbed on an adsorbent in an adsorbent cylinder filled with molecular sieve zeolite such as 5A type, 13X type, Li-X type, etc. as the adsorbent which preferentially adsorbs nitrogen over oxygen under a pressurized condition by supplying air compressed using a compressor, and a depressurization/desorption step in which the adsorbent is regenerated by reducing the pressure in the adsorbent cylinder to atmospheric pressure or lower and purging nitrogen adsorbed on the adsorbent.
Such an oxygen concentration device is mostly intended to be placed relatively close to the patient and is used continuously throughout the day by the patient regardless of dining or bedtime. Therefore, a noise generated from the oxygen concentration device is directly heard by the patient or patient's family, etc. and may give unpleasant feeling to them. There is concern that the noise has a significant influence particularly during sleep and the like by preventing the patient or the family from sleeping and thus negatively affecting their mental health. Sources of the noise generated by a pressure-swing absorption type oxygen concentration device include a structure-borne sound from a compressor for pressure variation, a suction sound and an exhaust sound from the compressor, an operating noise from a motor for driving the compressor, a purge gas flow noise of an adsorbent cylinder, and an operating noise from a cooling fan for the interior of the device housing. Among them, noises originated from the compressor such as the structure-borne sound radiation by the compressor, suction sound and exhaust sound of the compressor account for a large proportion of the whole device.
As described above, muffling the noise sources of the compressor is vital for the oxygen concentration device. A sound muffler called a cavity type or an expansion-chamber type has been used to reduce gas flow sounds such as the suction sound of the compressor and the like in a conventional oxygen concentration device. In addition to this, the sound muffler also needs to be miniaturized to realize a strong market demand of downsizing and weight reduction of the oxygen concentration device. For example, in a sound muffler described in Japanese Unexamined Patent Application Publication No. H10-245203, space saving is achieved by eliminating dead space by shaping the sound muffler into a cuboid and by integrating the sound muffler into an attachment part of the air filter.
However, the expansion-chamber muffler described above has such a feature that, as the ratio of the spatial cross-sectional area increases, the attenuation rate of the emitted sound increases, and the length of the cavity is related to the frequency of the sound desired to be reduced. Therefore, the physical size of the cavity is determined by the frequency band and the attenuation rate of the noise desired to be reduced, which is one of the obstacles for downsizing and weight reduction of the body of a low-noise oxygen concentration device.
In order to perform downsizing and weight reduction, while increasing the sound reducing effect of the sound muffler, Japanese Unexamined Patent Application Publication No. 2003-235982, for example, discloses a technique in which downsizing is maintained while serving sound attenuation by bringing an expansion-chamber muffler and a resonance muffler together and placing each muffler into the dead space.
As a sound muffler for attenuating a noise in high frequency band and low frequency band based on different principles, such technique as described in Japanese Unexamined Patent Application Publication No. 2005-6731 has been disclosed. In this technology, two sound mufflers are provided to reduce noises in high frequency band and low frequency band, where a long flow path is constituted using a sound-absorbing material for reducing the former, and this sound muffler is in turn incorporated in an expansion-chamber muffler for reducing the noise in the low frequency band, thereby achieving sound attenuation as well as downsizing and weight reduction. However, the expansion-chamber muffler requires the same size as before to reduce the noise in the low frequency band, and the fundamental technical problem remains unsolved. Furthermore, a conventional sound muffler needs to be connected to an intake port of a compressor, which is a vibrating body, through a pipe. Therefore, it becomes necessary to secure a structure, a space, and the like to isolate vibration, and there also arises a problem of space saving.